The invention relates to a magnetic focusing/deflection system for electron beam tubes, particularly camera tubes, and wherein focusing and deflection coils are provided which are electrically independent of one another.
By means of the focusing properties of the electron optics of known electron beam tubes, particularly of camera tubes working according to the Vidicon principle, differences in the sharpness of an image to be written or, respectively, scanned can arise between the center and edge of the screen.
In the middle of the screen, the optics of the magnetic main lens in general do not limit the resolution, since other influences predominate. The attainable limiting resolution in the center of the screen is determined in camera tubes essentially by the noise limit of the succeeding amplifier which sets a lower limit per image element for the current transported by means of the electron beam and thus determines the size of the image element to be resolved with a given beam current density. As is known, the attainable beam current density is subject to thermodynamic laws and cannot lie above the emission density of the cathode in a collecting screen lying at cathode potential. The resolution attainable for the usual oxide cathode lies at about a radius of 10 .mu.m for an image element and determines the resolution in the center of the screen. Upon deflection of the electron beam, additional electron-optical resolution errors ensue, the avoidance of which has, up to now, not been satisfactorily attained in systems with compact construction. For the possible focusing of a beam with a given strength, a thermodynamically established minimum cross-section is specified which cannot be fallen below without having a part of the electrons reverse.
Optical image errors of the imaging electron optic system only have a limiting effect on the resolution when they exceed the thermodynamically specified minimum cross-section of the beam. In the center of the screen, optical image errors are without influence upon retention of the rotational symmetry. However, the resolution is dependent to a high degree on deviations from the rotational symmetry which are conditioned by the manufacturing. At the edge of the screen, the imaging errors are amplified as a result of the precessional motion of the electron beam in the superimposed focus and deflection fields.
Attempts to attenuate the precessional motion were already undertaken at the beginning of the Vidicon development, cf. for example, Proceedings IRE 28, 1940, P. 30; Proceedings IRE 35, 1947, P. 1273 (Both incorporated herein by reference). Therefore, the influence of the field course in longitudinal direction was theoretically investigated and it was found that upon distribution of the rise and the decay of the deflection field over a respective full round-trip period in the main field, the precessional amplitude becomes very small. This perception was employed in the construction of a return-beam Vidicon with increased resolution, cf. RCA Review, March 1970, p. 60 ff (incorporated herein by reference). This known Vidicon is relatively elaborate, because four full round-trip movements of the focusing in the field of the main lens lie between aperture and screen, which requires a length of 28 cm for this path.
The existing problem, therefore, could up to now be solved only by means of a spatial separation of the focusing and of the deflecting field, which is effectually possible in image reproduction tubes, but not in camera tubes with compact construction, cf. also for example, "Handbook of Wireless Communications Technology", Volume 5, Television Technology, first part, "Basics of Electronic Television", BERLIN-GOTTINGEN-HEIDELBERG 1956, Pages 582-612, particularly Pages 583-584: "The Orthicon" (incorporated herein by reference).